1. Field of the Invention
The present invention relates to a picture engraving method in which a picture is engraved by a graving tool in response a picture signal that is picked up when an original picture is scanned, and also relates to a certificate with an engraved picture of a photograph of face, a fingerprint, a signature, a logo and the like, such as an identity card, a security and the like.
2. Description of the Related Art
A picture engraved on a dark opaque thin sheet and presented in shades of gray may be used for a personal identification. Such an engraved picture is hard to alter or counterfeit compared with an ordinary picture. However, the engraved picture can be tampered with black ink and thus, still needs careful inspection before judging genuineness. Japanese Unexamined Patent Publication No. 52-29221 discloses a genuineness determination method in which parallel light beams are projected from a certain angle to an engraved picture that is made by engraving a picture on a opaque thin sheet using a sharp-pyramid-like pointed graving tool and only reflected light beams having a characteristic directivity corresponding to the angle of the graving tool are searched. Furthermore, Japanese Unexamined Patent Publication No. 6-15794 discloses an engraving sheet made of an engraved non-light-transmissive film and a light-transmissive film.
In the first disclosure above, the shape of the graving tool should satisfy a predetermined requirement, and a special device is needed to verify a visual observation angle. In the second disclosure, an identical picture may be obtained by peeling off the non-light-transmissive film, applying white ink, and forming its negative. In such an engraving method, the back side of the sheet available for printing is narrowed.